Relax requirements on function modifiers in multiplatform projects

Allow to implement header functions with external/tailrec/inline/infix/operator
functions

compiler/testData/diagnostics/tests/multiplatform/topLevelFun/functionModifiers.kt

@@ -0,0 +1,19 @@
+// !LANGUAGE: +MultiPlatformProjects
+// !DIAGNOSTICS: -NOTHING_TO_INLINE
+// MODULE: m1-common
+// FILE: common.kt
+
+header fun external()
+header fun tailrec()
+header fun inline()
+header fun String.unaryMinus(): String
+header fun String.and(other: String): String
+
+// MODULE: m2-jvm(m1-common)
+// FILE: jvm.kt
+
+impl external fun external()
+impl tailrec fun tailrec(): Unit = if (true) Unit else tailrec()
+impl inline fun inline() {}
+impl operator fun String.unaryMinus(): String = this
+impl infix fun String.and(other: String): String = this + other

compiler/testData/diagnostics/tests/multiplatform/topLevelFun/functionModifiers.txt

@@ -0,0 +1,18 @@
+// -- Module: <m1-common> --
+package
+
+public header fun external(): kotlin.Unit
+public header fun inline(): kotlin.Unit
+public header fun tailrec(): kotlin.Unit
+public header fun kotlin.String.and(/*0*/ other: kotlin.String): kotlin.String
+public header fun kotlin.String.unaryMinus(): kotlin.String
+
+
+// -- Module: <m2-jvm> --
+package
+
+public external impl fun external(): kotlin.Unit
+public inline impl fun inline(): kotlin.Unit
+public tailrec impl fun tailrec(): kotlin.Unit
+public infix impl fun kotlin.String.and(/*0*/ other: kotlin.String): kotlin.String
+public operator impl fun kotlin.String.unaryMinus(): kotlin.String

compiler/testData/diagnostics/tests/multiplatform/topLevelFun/inlineFun.kt

@@ -15,4 +15,4 @@ impl fun nonInlineFun() { }
 // FILE: js.kt
 
 impl <!NOTHING_TO_INLINE!>inline<!> fun inlineFun() { }
-<!IMPLEMENTATION_WITHOUT_HEADER!>impl<!> <!NOTHING_TO_INLINE!>inline<!> fun nonInlineFun() { }
+impl <!NOTHING_TO_INLINE!>inline<!> fun nonInlineFun() { }